This invention relates to the use of electronic display materials for electric paper applications. The invention is designed for use with Gyricon electric paper but may also be used with electric paper based on liquid crystal, electrophoretic, and other field-effect display technologies.
Electric paper can be defined as any electronically-addressable display medium that approximates paper in form and function. Electric paper should be light-weight, thin and flexible, and it should display images indefinitely while consuming little or no power. In addition, electric paper should be re-usable. One must be able to erase images and create new ones repeatedly. Preferably, electric paper should display images using reflected light and allow a very wide-viewing angle.
One way to make electric paper possible using traditional electronic display technology is to completely remove the driving electronics from an electronic display package and use external addressing electrodes to write and erase images. This approach both reduces the per unit cost of electronic paper sheets and enables the use of cheap, flexible plastic films in place of glass plates for packaging. Multiple electronic paper sheets can then be addressed by a single set of external driving electronics, much like multiple sheets of pulp paper are printed on by a single printer.
A sheet and display system dubbed Gyricon is disclosed in various patents and articles, such as U.S. Pat. No. 4,126,854 by Sheridon titled "Twisting Ball Display." The Gyricon display system is comprised of an elastomeric host layer a few mils thick which is heavily loaded with rotating elements, possibly spheres, tens of microns in diameter. Each bichromal rotating element has halves of contrasting colors, such as a white half and a black half. Each bichromal rotating element also possesses an electric dipole, orthogonal to the plane that divides the two colored halves. Each bichromal rotating element is contained in its own cavity filled with a dielectric liquid. Upon application of an electric field between electrodes located on opposite surfaces of the host layer, the rotating elements will rotate depending on the polarity of the field, presenting one or the other colored half to an observer.
A Gyricon sheet has many of the requisite characteristics of electric paper, namely, bistable image retention, wide viewing angle, thin and flexible packaging, and high reflectance and resolution. U.S. Pat. No. 5,389,945 issued to Sheridon on Feb. 14, 1995, and titled "Writing System Including Paper-Like Digitally Addressed Media and Addressing Device Therefor", describes an electric paper printing system that employs independent, external addressing means to put images on the Gyricon sheets. The external addressing means is described as a one-dimensional array of electrodes connected, either directly or by wireless technology, to modulating electronics. As the one-dimensional array is scanned across the sheet, modulating electronics adjust the potential at the individual electrodes, creating electric fields between the electrodes and an equipotential surface. An image is created in the sheet according to the polarity of the electric fields. The patent recognizes that fringing fields in the vicinity of the addressing electrodes cause incomplete or excessive rotation of the imaging elements in the sheet, and it describes a method for solving this problem.
FIG. 1 shows a representation of the fringing field problem described in the '945 patent. A Gyricon sheet comprised of a plurality of bichromal rotating elements, in this case spheres, cast in a retaining medium 100 is contained between a first encapsulating layer 102 and a second encapsulating layer 104. The sheet 100 and encapsulating layers 102, 104 are placed in proximity to a supporting back plane 106 that is electrically grounded. An external addressing device 108 connected to a power supply 110 is depicted moving across the sheet in a direction D. Electric field lines 112 are shown in the proximity of the external addressing device 108 and bichromal spheres 126, 132, 138 within the electric field 112 are rotated to positions such that the planes that separate their black hemispheres 128, 134, 140 and white hemispheres 130, 136, 142 are orthogonal to the field lines 112. Note that a bichromal sphere 120 that was previously within the electric field 112 maintains a position similar to the bichromal sphere 126 in the trailing edge of the electric field 112. Optical properties of both of the bichromal spheres 120, 126 are not optimal because of their over-rotated orientations. A bichromal sphere 144 not yet affected by this electric field 112 rests in a state where its optical properties are optimized because its white hemisphere 146 is positioned precisely toward an encapsulating layer 102 which is also a viewing window.
FIG. 2 shows a representation of the return-to-zero effect, a problem not heretofore described, that limits the ability to address Gyricon sheets with external addressing devices as described in the '945 patent. A Gyricon sheet comprised of a plurality of bichromal rotating elements cast in a retaining medium 200 is contained between a first encapsulating layer 202 and a second encapsulating layer 204. The sheet 200 and encapsulating layers 202, 204 are placed in proximity to a supporting back plane 206 that is electrically grounded. An external addressing device 208 connected to a power supply 210 is depicted moving across the sheet in a direction D. Each bichromal sphere 220, 226, 232 is contained in its own liquid-filled cavity 221, 227, 233 within the retaining medium 200. Positive mobile ionic charge 240 and negative mobile ionic charge 242 are present in the liquid-filled cavity as well. An electric field exists directly between the external addressing device 208 and the equipotential surface 206 that causes the local bichromal sphere 226 to rotate and mobile ionic charges 240, 242 to separate within the cavity 227. In cavities 221 of regions trailing the path of the external addressing device and no longer under the influence of an external electric field, yet separated mobile ionic space charges create an electric field opposite to the previously applied field which imparts torque on the bichromal rotating elements 220 contained therein. This torque can dislodge the bichromal sphere 220 from its intended position, determined by the external addressing device, leaving its black half 224 and white half 222 in optically-poor position for viewing.
Another issue, heretofore undisclosed, facing electric paper printing systems is that sheets, once printed on by some external addressing device, are subject to inadvertent tribo-electric writing. In the described electric paper printing system, images are produced willfully by an external addressing device that has the ability to create electric fields. Electric charge applied inadvertently by tribo-electric exchanges during handling can equally create electric fields that cause image change. This effect poses a threat to image retention and stability. It should be emphasized that this significant problem is a threat to any electric paper technology which uses field-addressed electric paper sheets including Gyricon, liquid crystal and electrophoretic technologies.
A final issue facing the use of external addressing devices on electric paper sheets is that one-dimensional external addressing devices are limited in how quickly they can print an image on an entire sheet by the response speed of optical display elements. In Gyricon sheets, complete rotation of bichromal rotating elements is only achieved if the addressing electric field is held at least as long as the required rotation time, on the order of 100 milliseconds. For a sheet on which many rows of an image must by printed it would take many seconds or minutes to display an entire image.
Another issue facing electric paper is the difficulty of producing color versions. U.S. Pat. No. 5,717,515 by Sheridon issued Feb. 10.sup.th, 1998 and titled "Canted Electric Fields for Addressing a Twisting Ball Display" describes several methods for making highlight color and full color versions of a gyricon sheet and display. These systems all require multi-segmented spheres instead of bichromal spheres. That is the rotational elements needed to implement a color system have at least three different segments instead of the two segments used in the bichromal spheres. While production of multi-segmented spheres is possible the fabrication techniques needed are more complex and therefor the multi-segmented spheres are more difficult to manufacture than bichromal spheres. Additionally, these implementations use the complex addressing techniques of canted fields, multithreshold multipass addressing or addressing requiring multiple electrode addressing layers. Canted field addressing requires the generation of electric fields that are not substantially perpendicular to the viewing surface while multithreshold multipass addressing requires the usage of spherical elements which rotate when different strengths of electric fields are applied. In short, all of these systems are more complex and more difficult to implement than typical gyricons using bichromal rotational elements.
The present invention provides an improved means of implementing the printing system described in U.S. Pat. No. 5,389,945. The invention provides an alternative solution to the fringing field problem as well as addressing the problems of the return to zero effect, inadvertent tribo-electric writing, and limited scanning speeds. Futhermore, this invention provides a method for implementing improved grey scales, highlight color, and full color gyricons which use only simpler bichromal elements and do not require rotating elements which respond to different electric fields or canted field addressing.
Further advantages of the invention will become apparent as the following description proceeds.